I
have one answer that's short,
and
another that's longer, but less satisfying.

The short answer is: Probably not as
much as you think it does.

The
less satisfying answer is that size is somewhat
important, but there are plenty of things that matter more. If you're
a good cook or you have a charming personality, then congratulations,
you've probably compensated for your small penis and then some,
before you even reach the bedroom. And even in the bedroom, there are
things you can (and should) do to please your girl that don't involve
your wang at all.

I did a little
research on the statistics, in preparation for answering this
question. I won't bore you by repeating them, but it turns out that
the average measurements for penis length and girth are lower numbers
than you might expect. Men tend to underestimate their own size
relative to other men, so what you think of as small might not be
small at all. The bottom line is, there are a lot more men in this
world worrying about their penis size than there are women
complaining about it.